Methods used to separate cesium and other heavy alkali metal ions (K, Rb, Fr) from aqueous solutions include precipitation, solvent extraction and sorption on solid adsorbents or ion exchangers. Reagents used to precipitate such ions from solution have included sodium perchlorate, sodium chloroplatinate, sodium tricobalinitrite and sodium tetra-aryl boron compounds such as sodium tetraphenylboron (Hogness, Johnson and Armstrong, Qualitative Analysis and Chemical Equilibrium, 5th Edition, 1966, pp. 349-359). These compounds can be used to separate heavy alkali metal ions from the aqueous phase either by filtration or centrifugation of a precipitated solid or by means of solvent extraction of the product into an organic phase in which it is more highly soluble than in water.
For instance, U.S. Pat. No. 4,432,893 describes a process for the removal of the residual hazardous soluble values from a nuclear waste solution which comprises contacting the solution with sufficient sodium tetraphenylboron to precipitate the cesium present. Potassium is also precipitated. Simultaneously, sufficient sodium titanate is added to form an insoluble slurry which sorbs strontium and removes it from the waste supernate. The waste solution is then filtered to separate the solution substantially decontaminated of the hazardous cesium and strontium values.
The filtration step can be avoided. For instance, U.S. Pat. No. 2,982,785 describes a process of separating cesium values from an aqueous solution which comprises adding a sodium tetraphenylboron solution in hexone (preferably at a concentration of about 0.1M) to the aqueous cesium solution (which preferably has a pH value of from 2 to 13) whereby cesium tetraphenylboron is formed and taken up in a hexone phase, and separating the hexone phase from an aqueous raffinate.
The main application of the various techniques to remove cesium from aqueous solutions is the decontamination of streams in the nuclear industry which contain the radioactive isotopes Cs-134 (.tau..sub.1/2 =2.05 years), Cs-135 (.tau..sub.1/2 =3.times.10.sup.6 years), CS-136 (.tau..sub.1/2 =13 days) and CS-137 (.tau..sub.1/2 =30.2 years). These radioactive species are present in solution in the form of the Cs.sup.+ ion. However, in the nuclear industry one often encounters solutions which cannot be decontaminated by means of materials which have been used to remove cesium ions from solutions such as organic cation exchangers, zeolites and silica-based glasses and gels such as the Durasil-10 ion-exchanger. For instance, one of the major fission product streams obtained in re-processing irradiated fuel at the Savannah River Plant, Department of Energy, will contain approximately 7.1M Na.sup.+, 0.015M K.sup.+ and 0.0004M Cs.sup.+, and the pH will be approximately 14 as the result of the presence of 1M NaOH and of other basic salts. Under these conditions, the exchange capacity of organic ion-exchange resins for cesium is very low because of the high content of other alkali ions, while silicates and aluminosilicates undergo very rapid corrosion in the highly alkaline medium.
Accordingly, it is an object of this invention to provide a process for the recovery of heavy alkali metal cations from liquids which is simple and does not require any special equipment.
It is also an object of this invention to provide a process for the selective recovery of heavy alkali metal cations from liquids that contain more than one heavy metal cation and particularly for the selective recovery of cesium from liquids that contain salts other than cesium in high concentrations and the cesium in substantially lower concentrations.
It is further an object of this invention to provide a process for the recovery of heavy alkali metal cations from aqueous solutions which can be carried out from alkaline as well as acid solutions.